This invention relates to an antenna drive. In one conventional type of antenna drive system the antenna is caused to scan repetitively through an arc of 80 or 90 degrees by a longitudinal screw threaded drive member which is moved to and fro by an electrically operated drive mechanism engaging with the screw thread. In such systems the screw threaded drive member is pivoted to the antenna structure and the drive is pivoted to a fixed mounting structure so as to accommodate relative turning movements between those parts during operation of the drive member. It is sometimes desired to adjust the mounting of the antenna so that it scans through a different arc. Hitherto, in order to effect such adjustment it has been necessary to unbolt the antenna drive from its mounting and rebolt it at an alterhative position. This is obviously a very time consuming exercise and it is of course impracticable to make such adjustment whilst tracking a target which may move from one of the said arcs to the other. It was with this problem in mind that the present invention arose.